Dynamic range enhancement technique

ABSTRACT

Apparatus and methods are disclosed for adding minimum pulse widths to the coarse resolution output of a multi-reference switching amplifier. This acts to null any detectable resultant differential by addition of a dynamic offset pulse width to the fine resolution output; thus extending operation to zero while presenting the imposed minimum pulse widths as a common-mode (null) signal. A preferred method according to the invention includes the steps of separating the input signal into coarse and fine resolution outputs, adding a minimum pulse width to the coarse resolution output; and adding a dynamic offset pulse width to the fine resolution output so as to null differential signals which would otherwise be present across the load. In terms of circuitry, a data separator is employed for separating the input signal into coarse and fine resolution outputs, and a summer adds the minimum pulse width to the coarse resolution output. Pulse-width modulation converters transform the modified coarse- and fine-resolution outputs prior to presentation to the load through routing logic.

REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. provisionalapplication Ser. No. 60/225,222, filed Aug. 14, 2000, the entirecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to electronic amplifiers and, inparticular, to apparatus and methods for enhancing the dynamic range ofmulti-reference switching amplifiers.

BACKGROUND OF THE INVENTION

[0003] Many electronic devices operate on the principle of providing theintegral of two or more switched voltage or current levels as an analogoutput. In order to modulate this analog output, the effective dutycycle of the switched voltages or currents is changed by various meansin accordance with an input source. In such devices, predictablemonotonic resolution is attainable only at pulse widths above theminimum imposed by the aggregate propagation delays of the physicalsystem. Below this minimum, pulse widths obtained do not correlatelinearly to their commanded width.

[0004] Multiple-reference switching amplifiers enhance resolution byswitching two or more reference voltages to a load. Configurations ofthis type are described in patent application PCT/US99/26691, entitled‘Multi-Reference, High-Accuracy Switching Amplifier,’ the contents ofwhich are incorporated herein by reference.

[0005] Although the addition of a second reference voltage improvesresolution, it does not directly address resolution near zero output,where propagation delays impact crossover distortion. In manyapplications, such as audio amplification, dynamic range requirementsoften make operation below this minimum desirable. Accordingly, a needexists to extend operation of these switching devices to facilitateresolutions significantly lower than their propagation delay.

SUMMARY OF THE INVENTION

[0006] This invention is directed to apparatus and methods for addingminimum pulse widths to the coarse resolution output of amulti-reference switching amplifier, and nulling any detectableresultant differential by addition of a dynamic offset pulse width tothe fine resolution output; thus extending operation to zero whilepresenting the imposed minimum pulse widths as a common-mode (null)signal. Although the technique is described with reference to thecoarse-resolution signal, it is equally applicable to thefine-resolution signal, or both.

[0007] A preferred method according to the invention includes the stepsof separating the input signal into coarse and fine resolution outputs,adding a minimum pulse width to the coarse resolution output; and addinga dynamic offset pulse width to the fine resolution output so as to nulldifferential signals which would otherwise be present across the load.

[0008] In terms of circuitry, a data separator is employed forseparating the input signal into coarse and fine resolution outputs, anda summer adds the minimum pulse width to the coarse resolution output.Pulse-width modulation converters transform the modified coarse- andfine-resolution outputs prior to presentation to the load throughrouting logic.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a block diagram of a prior-art bridged-output Class D(PWM) amplifier;

[0010]FIG. 2 shows the timing of signals normally encountered in thecircuit of FIG. 1 as a low-level signal traverses zero;

[0011]FIG. 3 is a circuit diagram of a multi-reference, bridged-outputswitching amplifier incorporating techniques according to the invention;

[0012]FIG. 4 shows the timing of signals resultant from the presenttechnique applied to the amplifier of FIG. 3 as a low-level signaltraverses zero; and

[0013]FIG. 5 is a diagram depicting circuitry associated with thepulse-width modulator of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

[0014]FIG. 1 is a block diagram of a prior-art bridged-output Class D(PWM) amplifier. Pulse-width modulator 102 converts incoming data stream101 into pulse-width modulated drive signals 111, 112, 113, and 114,which drive switching devices 103, 104, 105, and 106, respectively.Inductors 107 and 108, in conjunction with capacitor 109, filterswitching components from the outputs of said switching devices, andsupply analog output to load 110. Operation of this type of amplifier iswell known in the art.

[0015] Referring now to FIG. 2, traces 201 and 202 show inputs toswitching devices 103 and 104, respectively, and trace 205 shows outputfrom switching devices 103 and 104 to inductor 107, all of FIG. 1. Trace203 and 204 show inputs to switching devices 105 and 106, respectively,and trace 206 shows output from switching devices 105 and 106 toinductor 108, again all of FIG. 1. At small pulse widths, it can be seenthat outputs do not accurately represent inputs; output pulses may beelongated or missing.

[0016]FIG. 3 is a circuit diagram of a multi-reference, bridged-outputswitching amplifier incorporating techniques according to the invention.Pulse-width modulator 302 converts incoming data stream 301 intopulse-width modulated drive signals 313, 314, 315, 316, 317, and 318,which drive switching devices 303, 304, 305, 306, 307, and 308,respectively. Inductors 309 and 310, in conjunction with capacitor 312,filter switching components from the outputs of the switching devices,and supply analog output to load 311.

[0017] Switching devices 303 and 306 source V+ to inductors 309 and 310,respectively, when activated. Switching devices 304 and 307 sourceV_(r), which is substantially lower than V+, to the inductors,respectively. Switching devices 305 and 308 connect the inductors,respectively, to ground in the absence of competing control signals. Thebasic operation of this type of amplifier is described in pendingapplication PCT/US99/26691 referenced above.

[0018] According to this invention, operational amplifier 319 receivesas input the differential imposed across load 311, and supplies feedbacksignal 320 to pulse-width modulator 302. Traces 401, 402, 403, 404, 405,and 406 of FIG. 4 show the inputs to switching devices 303, 304, 305,306, 307, and 308, respectively, of FIG. 3. Traces 407 and 408 showoutput voltages from switching devices 303, 304, and 305 to inductor309, and output voltages from switching devices 306, 307, and 308 toinductor 310, respectively, all of FIG. 3. Trace 409 represents thedifference between traces 408 and 407, the filtered version of which isultimately presented to load 311 of FIG. 3.

[0019] At time marker 410, a pulse width calculated by pulse-widthmodulator 302 from the incoming data value, plus a minimum pulse width,activates switching device 303. Concurrently, a minimum pulse widthactivation of switching device 306 is output. In this sample, theincoming fine data is assumed to be zero, so no activation occurs intrace 406. The net output seen in trace 409 for this sample period istherefore (coarse data value*V+).

[0020] At time marker 411, the coarse data value is zero, so concurrentminimum pulse widths are output to both switching devices 303 and 306.The fine data value is non-zero, such that a pulse width consisting ofthe fine data value then activates switching device 308, as seen intrace 406.

[0021] The net output seen in trace 409 for this sample period istherefore (fine data value*Vr). At time marker 412, a smaller fine datavalue is received, with a coarse data value of zero. Resultantly, thenet output seen in trace 409 for this sample period is a shortenedversion of the previous period between markers 411 and 412. At timemarker 413, the sign of the incoming data changes, and a small fine datavalue is received with coarse data value of zero.

[0022] As expected, the minimum pulse widths presented to switchingdevices 303 and 306 are followed by a pulse width representative of theincoming fine data value at switching device 305, as seen in trace 403.However, a following pulse is as well output to switching device 308,seen in trace 406. This is an offset value added by pulse widthmodulator 302, in response to previous input from operational amplifier391 indicating a need for offset correction in the indicated polarity inorder to obtain a null output at zero.

[0023] Note that this offset pulse width (seen in trace 406) is appliedfor all outputs of this polarity after time markers 414 and 415 as well.The net outputs seen in trace 409 for the samples between markers 413and 414, and markers 414 and 415, are therefore (offset value*Vr)−(finedata value*Vr). The net output seen in trace 409 for the sample atmarker 415 is (offset value*| Vr)−(coarse data value*V+), since the finedata value is zero, indicated by the lack of assertion in trace 403.

[0024] Referring now to FIG. 5, incoming data stream 501 is input toabsolute value generator 504, the output of which is input to dataseparator 506, which separates incoming data into coarse and fineresolution data streams. Resultant coarse data is summed with a minimumvalue 505 by summer 509 before application to pulse width converter 513.The minimum value is also applied to pulse-width converter 512 as input.The fine data output from data separator 506 is input to pulse-widthconverter 514.

[0025] Feedback signal 503, received from operational amplifier 319 ofFIG. 3, is applied to integrator 507, the output of which is deliveredto absolute value generator 511, which supplies data to pulse-widthconverter 515.

[0026] The output sampling rate of the system is driven by clock 508,which triggers pulse-width converters 512 and 513. Pulse-widthconverters 514 and 515 are triggered by the de-assertion of the outputof pulse width converter 513.

[0027] The operation just described results in pulse-width train 517,representative of minimum value 505 and pulse width train 518,representative of the coarse data value plus minimum value 505; followedby pulse width train 519, representative of the fine data value, andpulse width train 520, representative of the absolute value ofintegrator 507. Being driven by a feedback signal, integrator 507provides an offset correction factor discussed with reference to FIG. 4that moves the amplifier output toward an average of zero across load311 of FIG. 3.

[0028] Pulse-width trains 517, 518, 519, and 520, along with the signindications of incoming data stream 501 and integrator 507, are appliedas input to output routing logic 521, which routes said pulse widthtrains to switching device control signals 522, 523, 524, 525, 526, and527, which correspond to signals 313, 314, 315, 316, 317, and 318 ofFIG. 3, respectively. Functional description of said routing logic 521is available in patent application referenced above, but may as well beseen in FIG. 4.

[0029] Feedback data may be either qualitative or quantitative. Inaddition, although voltage references are disclosed, it will be apparentto one of skill in the art that equivalent operation using currents ispossible through appropriate circuit modification.

I claim:
 1. A method of enhancing the dynamic range of a switchingamplifier of the type wherein multiple references are switched to a loadin accordance with an input signal, the method comprising the steps of:separating the input signal into coarse and fine resolution outputs;adding a minimum pulse width to the coarse resolution output; and addinga dynamic offset pulse width to the fine resolution output so as to nulldifferential signals which would otherwise be present across the load.2. Apparatus for enhancing the dynamic range of a switching amplifier ofthe type wherein multiple references are switched to a load inaccordance with an input signal, comprising: a data separator forseparating the input signal into coarse and fine resolution outputs; aminimum pulse width generator; a summer for adding the minimum pulsewidth to the coarse resolution output; and circuitry for adding adynamic offset pulse width to the fine resolution output; and convertersfor pulse-width modulating the modified coarse- and fine-resolutionoutputs for presentation to the load.